1. Field of the Invention
The present invention relates to interconnecting multichip modules and more particularly to forming electrically conductive metal bonds between integrated circuit devices.
2. Description of Previous Art
As advances in processing technology allow ever increasing number of integrated components to be fabricated on an integrated circuit (IC), The industry is constantly striving to integrate more circuit devices for each packaged integrated circuit.
Often times, the speed at which a system can operate is limited by resistor-capacitor (RC) delay and crosstalk coupling of one signal into another. The RC delay and crosstalk coupling problems are even more prominent as IC devices becomes smaller.
One technique that allows more circuit devices and more performance from a packaged IC is through the use of multichip modules (MCMs) technology. MCMs integrates a plurality of individual modules of circuit devices formed on a single IC die. MCMs have the advantage of increasing yields of highly complex ICs by piecing a complex IC together with smaller IC components. In this way, defects that do occur effect individual components of the MCM rather than a single large complex die. Defects affecting the larger dies are much more costly.
The IC modules of the MCM packages are bare ICs. Many of these ICs are fabricated on top of a monolithic silicon IC wafer and designed for perimeter wire bonding. Devices are first formed on the silicon IC wafer. Multiple metallization layers are formed on the bare IC to provide a connection medium from the IC to the outside world and within the IC. Aluminum is commonly used for the metallization layers because aluminum exhibits desirable properties such as ease of processing and low resistivity. The top most level metal is deposited and patterned during the last stages of processing, whereas the gates, contacts, and the metallization of the devices are formed during earlier stages. The top level metal is thick because it is a current carrier for the IC.
Many MCM designs have evolved and is continuing to develop. A basic MCM design has a structure having two or more IC modules electrically connected to an interconnect substrate. The interconnect substrate has patterns formed on multiple layers to provide the interconnects to the IC modules. The multiple layers are separated by dielectric to prevent shorting between the layers. Contacts formed on the IC modules and the interconnect substrate provide coupling points for first level connections which couple the IC modules to the interconnect substrate. Additional contacts formed on the interconnect substrate provide second level connection points which couple the interconnect substrate to external pins of the packaged MCM.
MCMs have fundamental advantages of increased speed, reduced number of external connections, and reduced overall size. The interconnects between the IC modules are substantially shorter than what would otherwise be traces on a printed circuit board. The short interconnects increase signal integrity and reduce crosstalk. The number of external connections to the packaged IC is also reduced. Signals between the IC modules of the MCM are interconnected within the MCM. Furthermore, the overall size of an MCM is reduced by integrating many IC modules into a single MCM package.
In order to achieve peak performance from a system of individual ICs, it is desirable to keep connections between the individual ICs as short a possible. Circuit speed on the bare IC level is the highest speed achievable for the IC. As soon as interconnects are introduced, circuit speed begins to suffer. In an MCM, it is advantageous to limit interconnects lengths between the ICs.
Wirebond and tape automated bonding (TAB) techniques are used to provide connections between the IC modules and the MCM substrate. These techniques use leads for carrying electrical signals from one IC module to another. Thus, wirebond and TAB techniques suffer from circuit speed degradation and crosstalk that are inherent in lead designs. Some MCM designs have addressed reduced interconnect lead problems by using a flip chip solder bump (FCSB) technology.
IBM first introduced the FCSB interconnections in 1964. FCSB interconnections are still widely practiced by IBM today. Bare ICs with solder bump terminals and a matching set of solder pads on a substrate are mated to produce interconnections between the ICs of the MCM. The ICs are placed upside down or flipped to mate with the substrate. In this way, interconnects between the IC modules are kept at a minimum. Solder bumps are used to attach the IC modules and the substrate. Each solder bump is aligned with it matching substrate pad. All the solder joints are processed simultaneously by reflowing the solder in a furnace.
There are however some disadvantages of the FCSB interconnections. The disadvantages arise from the connection medium between the flipped ICs and the underlying MCM substrate. The MCM substrate is typically much larger than the ICs. Multiple ICs are attached to the MCM substrate to form the MCM. The larger MCM substrates are more complex to build which reduces the yield of FCSB MCMs. Furthermore, because the MCM substrate functions to interconnect the IC modules, parasitic capacitance present in the larger and more complex substrate degrades electrical performance of the IC modules. Thus, FCSB interconnections adds cost and complexity to the fabrication of MCM modules.
Therefore, it is desirable to provide interconnects well suited for IC modules, and a method of operating the same which preserve bare IC circuit speed and performance without suffering from disadvantages of previous MCM designs.